Water contamination in aircraft fuel systems can cause many problems. In particular, water contamination in fuel is known to collate in aircraft fuel tanks. When an aircraft reaches altitude during operation, the ambient temperature drops to around −50° C. which can lead to ice formation. Water contamination in aircraft fuel systems typically occur due to water in fuel being fed into an aircraft fuel system upon the supply of fuel to the aircraft, or through venting in the fuel system.
The presence of water in a fuel tank can lead to the growth of microbiological contamination (MBC), which may lead to the clogging of fuel system components, such as filters, fuel pumps and gauges. The formation of ice during operation of an aircraft at altitude may also cause similar problems. As such, it is desirable to remove water from a fuel tank before such problems occur.
One known method for removing water from a fuel system is to locate a water drain valve at the base of a fuel tank. However, such water drain valves are typically unable to discriminate between water and fuel such that either insufficient water is drained from the fuel tank, or fuel is removed from the fuel tank together with the water. Moreover, it is necessary for an operator to manually operate the drain valve, or to use a complicated system to open and close the valve. Such a waste fuel/water mix should be disposed of in an appropriate manner.
WO2014/174247 describes a tank assembly with a tank for storing liquid hydrocarbon, the tank having a floor for supporting a weight of the liquid hydrocarbon. A filter is fitted to the floor of the tank. The filter in arranged to allow liquid water in the tank to drain out of the tank through the filter but substantially prevent the liquid hydrocarbon in the tank from doing so. The filter has a permeation member such as a membrane, which is formed from a material such as graphene oxide which allows liquid water in the tank to drain out of the tank by permeating through the permeation member but substantially prevents the liquid hydrocarbon in the tank from doing so.
“Unimpeded. Permeation of Water Through Helium-Leak-Tight Graphene-Based Membranes”, R. R Nair et al, Science, 27 Jan. 2012, Vol. 335, no. 6067, pp. 442-444, DOI:10.1126/Science.1211694 demonstrated that submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquid, vapours, and gasses, including helium, but these membranes allow unimpeded permeation of water.